The present invention relates to process control in general, and more particularly, to a computer technique for the optimization of process control through the use of a matrix.
Optimization techniques are known to maximize the production and/or to minimize the cost of operating an industrial process, especially with regard to the consumption of energy and the correlating production output. These techniques are particularly applicable in chemical engineering, and they have been used there for establishing operating conditions that yield a maximum return on investment while minimizing operating costs.
The prior art reveals mathematical optimization techniques, such as linear programming and evolutionary operation techniques. The latter has been disclosed in Chemical Engineering of July 5, 1965 "Process Improvement with SIMPLEX Self-Directing Evolutionary Operation" by B. H. Carpenter and H. C. Sweeney, pp. 117-126. The evolutionary, or EVOP, method of optimization has been applied ideally to a steam-power cogeneration process, as disclosed in patent application Ser. No. 550,164 filed Nov. 8, 1983, now U.S. Pat. No. 4,604,714 by treating cogenerated electrical power in terms of steam flow and using steam flow and power balances in the process to make a model readily applicable for successive tests leading to an optimal solution.
Another optimization technique is the linear programming approach which uses a matrix reflecting the key variables and parameters in the cogeneration process, also in terms of steam flow. In contrast to the EVOP model the linear programming matrix, however, is able to take into account more process variables, more parameters and still reflect the various possible status of the equipment such as MANUAL/AUTO ON/OFF, and handle many constraints.
The linear programming method is known in the prior art (see for instance: (1) Dantzing, G. B., "Linear Programming and Extensions", published by Princeton University Press, 1963; (2) Hadley, G., "Linear Programming", published by AddisonWesley Publishing Co., 1962; (3) Llewellyn, R. W., "Linear Programming", published by Holt, Rinehart and Winston, 1964; (4) McMillan, Claude, "Mathematical Programming", published by John Wiley and Sons, Inc., 1970.) Because of its very complexity linear programming has been thought to be impractical where optimization should lead to a prompt implementation of the solution. It is the reason why, in the past, the well known linear programming technique would never have been used in a cogeneration steam-power plant project. By the time the optimization system would seem to converge on the desired solution, the initial conditions taken into consideration by the calculations performed with the matrix would have or could have, changed so much that the mathematical solution would have lost any physical meaning and could not be implemented by control of the plant. The particular mass flow treatment exercised for linear programming, as well as in accordance with the EVOP approach disclosed in patent application Ser. No. 550,164 filed 11/8/83, now U.S. Pat. No. 4,604,714 makes it practical to use the linear programming approach for optimization in a cogeneration steam-power plant.
A linear programming matrix based upon base flow treatment of the steam and power as exemplified in the description hereinafter of the preferred embodiment, shown that the matrix can not only be reduced to a size which is easy to handle, and without ceasing to reflect the complexity introduced by noting equipment status and adding pressure and temperature to depict true mass flow, but by also leading to a matrix exhibiting so many zero coefficients, that the program code is able to jump around the zero elements in the matrix and converge to a solution within a time so short as to allow a direct implementation by control of the found solution. This is a favorable circumstance since it is desirable to be able to quickly estalish what the optimum result is, as soon as a number of initial conditions in the process have been ascertained. There may be a new steam demand, or a new power demand, in the plant, or an exercise of demand control in regard to the demand limit assigned to the tie-line.
In accordance with the present invention, optimization (for instance by the EVOP approach or the linear programming technique) is used for maximizing the utilization of steam and the generation of power in a plane of control, for instance where the steam is derived from a main stream of steam by a steam-power generating and processing installation, through at least a first and often a second steam-to-electrical power converter. The main stream of steam is distributed into first and second independently regulated steam flow inputs to the first and second converters, respectively. The first and second steam converters have independently regulated respective first and second steam flow outputs. The first and second converters have respective first and second steam-electrical power response characteristics for providing electrical power in a cogeneration mode in relation to the first and second steam flow inputs and outputs. Means are provided for controlling steam flows in the first and second inputs and outputs to generate electrical power and deliver steam at the first and second flow outputs in accordance with a predetermined power demand and steam demand of the installation. A tie-line network is provided for supplying complementary electrical power for said power demand at a cost different from the power derived from steam by cogeneration. Several global equipment status and network configurations of the system are anticipated, which define a number of predetermined and discontinuous planes. Such a plane for instance may correspond to all steam turbines being operative. Another plane would be when a particular turbine has been tripped. For each anticipated plane, whether present or not, the optimization process is conducted, off-line with a model involving balanced mass flow between the first and second steam flow inputs and outputs on the basis of an initial setting. As a result of such optimization, results are obtained which suppose that the first and second steam flow inputs and outputs will be given a particular setting in accordance with the values generated by the off-line optimization of the model. These results of the optimization are stored, from time to time refreshed to match intervening fluctuations in the demand of steam and power. Then, settings are implemented for the particular plane as soon as it becomes active, i.e. becomes the new present plane.
The off-line optimization typically is performed, with an EVOP model or by linear programming, with a matrix arranged on the basis of balanced inputs and outputs of steam and power only, thus, without the assist of enthalpy or entropy characteristics in determining the power derived by cogeneration.
Constraints are exercised, ON and OFF status of the equipment are recognized and consideration is given whether inputs and/or outputs are reaching inferior or upper limits during the optimization process, such limits being accepted as a constraint in the model response evaluation.
Taking advantage of such optimization approach for cogeneration of steam and power for several planes in advance, the invention makes use of the optimization approach off-line in a multiplane system, each plane involving mass flow of fluid, and corresponding to a different structural arrangement of flow paths, turbine valves, boilers, steam demand inputs and steam demand outputs, whereby different settings can be immediately established in the new plane and a quick use of another optimization process therein is possible.
Thus the object of the present invention is to make it possible to estimate the optimum assignment of steam flows and power throughout the system for any particular cogeneration system defined in a new plane for establishing an optimum distribution of the available steam matching the process steam demand and the power demand at the respective pressures therein.
The invention provides speedy and intelligent reaction to the various contingencies when leaving a control plane and reaching a new control plane by (1) providing a rapid and safe mode of switching between planes and (2) providing ahead of valve control at the junction between planes pre-planned responses to each of an anticipated set of contingencies with respect to switching to a new control plane. On the latter point, it is observed, incidentally, that a steam valve can be used normally as a pressure reducing valve and occasionally as a let-down valve, so as to hold the pressure in the inlet header to a safe value. The latter alternative is provided for safety considerations, since the valve inlet pressure would take precedence over the outlet pressure loop commands of the valve. The conventional approach in this respect has been, under an analog method, to provide two separate controllers, the higher of the two outputs thereof being selected to prevail. The two output signals would, in an alternative solution, be applied to a summer, so that the high-pressure controller would become predominant, as soon as its output signal has prevailed over the signal of the other controller. However, the switching operation, e.g. before either currently active controller assumes full control over the other, raises a question of timing. A sizable time inherent to the controllers will lapse, which is due to the time taken for the associated integrators to decay. Besides, in the intervening time interval, the controllers on either sides tend to react back and forth. Control plane switching according to the present invention eliminates such delays and interactions. The invention provides for a velocity algorithm to be used for feedback control, thereby allowing the position register for each valve to be adjusted from an external source, while assuming pre-calculated values, which are down-loaded to the registers as soon as such disturbances are occurring. Subsequently, in accordance with the technique of application Ser. No. 548,478 filed Nov. 3, 1983, now U.S. Pat. No. 4,577,280 normal execution of the (PID) controller algorithms will provide values for adjustment of the valve position, in order to bring any residual header pressure error within tolerances.
The first point mentioned above has to do with the initial phase leading to implementation within the selected new plane for optimization within a multiplane fluid and power distribution system.
Such contingency, for instance, involves in a steam cogeneration plant, the calculation ahead of time of the settings of valve position to which the plant should be moved in the event of one or more turbo-generators tripping off-line. It could also be the tie-line circuit breaker falling open.